Welding dissimilar metal members



Nov. 24, 1959 E. V. YUHASZ WELDING DISSIMILAR METAL MEMBERS Filed Jan. 2. 1958 l i 3' 3 2913 g INVENTOR EUGENE V.YUHASZ United States Patent 2,914,641 WELDING DISSIMILAR METAL MEMBERS Eugene V. Yuhasz, Fanwood, N.J., assignor to Union Carbide Corporation, a corporation of New York Application January 2, 1958, Serial No. 706,774

13 Claims. (Cl. 219-95) This invention relates to the production of a welded joint between dissimilar metal members, and more particularly to the production of a welded joint between a first metallic member and a second dissimilar metallic member, the latter havinga higher melting temperature and electrical resistivity, but lower thermal expansion coefficient than the first metallic member, as for example, a joint between aluminum and stainless steel.

Aluminum is often desirable as a construction material because of its properties of light weight, high thermal conductivity, and low electrical resistance. In many of its applications, however, undesirable results are obtained when joints must be made between aluminum and other metals. Such connections, for example, are required in the refrigeration industry when aluminum evaporator coils are joined to steel vessels. They are also required in oxygen supply systems for high altitude breathing, when a double walled aluminum liquid oxygen storage container may have stainless steel tubing communicating between the inner and outer vessels. In these two applications the dissimilar metal joint must be leak-tight and must retain its strength at low temperatures. Joints between aluminum and stainless steel or iron made by the prior art methods are unsatisfactory because of frequent failure caused by embrittlement of the aluminum-iron intermetallic compounds.

Many solutions to the dissimilar joint problem have been suggested. One proposed solution consists of placing an aluminum coating of controlled thickness on the steel article and then casting or brazing the aluminum article to the aluminum coating. Thismethod provides joints which may be useful at temperatures below about 600 F. and under relatively low flexing conditions. However, such joints fail under more rigorous conditions due to the presence of the brittle aluminum-iron alloy at the joint interface.

Another proposed solution to the joint embrittlement problem utilizes a silver intermediate layer. method, the steel is plated with a desired thickness of silver. The aluminum is then placed in contact with the silver plating and the joint is formed by electrical resistance welding combined with high pressure. A portion of the silver-diffuses into alloys with the aluminum to provide a strong aluminum-silver-iron bond without formation of a brittle aluminum-iron alloy. However in this process, another serious problem exists. It is extremely difficult in a resistance welding system to arrive at an'ideal joining temperature for all the metals, since aluminum, silver and the other joint materials all have different melting points. For example, aluminum has a-relatively low melting point (about 1200" F.) compared to stainless steel (above 1900" F.) and silver (about 1760 F.). Another problem which must be faced is the diiferent coefficients of thermal expansion, aluminum having a higher coefficient than stainless steel. Resistance welding is also limited in application to parts wherein large concentrated pressures can be applied and backed up. It is impractical to use such a resistance In this welding process for joining slender, thin-walled tubes because they cannot withstand the required high pressures nor can back-up electrodes be inserted inside such tubes.

One object of the present invention is to provide a highly efficient process for welding together a first metallic member such as aluminum and a second metallic member such as stainless steel, which process avoids the formation of aluminum-iron intermetallic compounds which cause weld failure by embrittlement.

Another object is to provide a process for welding together thin tubes of aluminum and stainless steel, which process does not require the application of high pressures which would distort the thin walls of the dissimilar metal through two dissimilar metal tubes during the first step of a process for welding such tubes together, according to the present invention;

Figure 2 is a view of a longitudinal cross-section through the same tubes as shown in Figure 1, during the intermediate step of the welding process;

Figure 3 is a view of a longitudinal cross-section through the same tubes as shown in Figures 1 and 2, at the completion of the subject welding process; and

Figure 4 is a view of an enlarged longitudinal crosssection through the dissimilar metal joint of Figure 3.

According to the present invention a process is provided for welding together by high-frequency induction heating, a first metallic member having a hollow section of given contour and a second dissimilar metallic member provided with a projection having a contour and size corresponding to the hollow section. The first member may for example be constructed of aluminum and the second member formed of stainless steel. The aforementioned projection on the stainless steel member is first coated with an intermediate bonding material such as silver, which has a melting temperature between the melting temperatures of the aluminum and stainless steel members and which forms an alloy with stainless steel. The external dimensions of the coated projection and the hollow section are such that the former is slightly larger than the hollow section at ambient temperature, but the coated projection fits tightly in such section when the latter is heated. A high frequency magnetic field is established adjacent to the aluminum member so as to directly heat such member to the desired joint forming condition. Due to thermal expansion its hollow section soon reaches a size which permits insertion of the silver coated stainless steel projection therein. The induction heating is continued until a desired amount of the intermediate bonding silver has melted and diffusion taken place between the silver and the aluminum to form a stable silver-aluminum alloy. Since stainless steel has a higher electrical resistivity than aluminum, it will heat up more rapidly and therefore is heated for a shorter time period than the aluminum member.

In a preferred embodiment of the present invention, a silver-coated stainless steel tube end is placed in contact with an aluminum tube during the heating step. In this way, the aluminum tube preheats the stainless steel tube by conduction. Since aluminum has a higher thermal expansion coefiicient than stainless steel and the heat is applied directly thereto, it expands at a faster rate than the stainless steel tube and still allows such tube to be inserted therein.

The aforedescribed process is particularly suited for joining dissimilar metal members in the form of tubes because it requires the application of relatively little pressure, thus avoiding the high pressures used in conjunction with resistance-welding techniques, which could distort thin metal tubes. When aluminum and stainless steel tubes are to be joined according to the present invention, the smaller diameter stainless steel tube is coated with silver on the joint end, and the aluminum tube is either spun or reamed out so as to form a light press fit over the silver-plated stainless steel tube, when subsequently heated. The aluminum tube is then placed in a high frequency induction coil and the silver-coated stainless steel tube is preferably placed in contact with one end of the aluminum tube but not inside the heating field of the induction coil. The heating unit is then turned on and the joint is formed in the previously described manner.

It is to be noted that if a press fit joint of aluminum and silver-coated stainless steel were placed directly in the induction coil and heated, the stainless steel would be overheated and the silver would have melted away before the aluminum reached fusion temperature, due to the lower electrical resistivity of aluminum. The present invention provides a method of avoiding this problem; that is, by introducing different quantities of heat to the dissimilar metals and thereby realizing ideal joint formation conditions. This invention also advantageously utilizes the difference in thermal expansion coefficients of aluminum and stainless steel. Before heating, the cold joint materials have such dimensions that the silver coated stainless steel projection could not slip into the hollow section of the aluminum member. As the aluminum expands due to heating, the stainless steel projection can be conveniently forced into the hollow aluminum section. When the joint cools, the contraction of the aluminum strengthens it, which aids in the production of a strong leak-tight joint.

The present invention also avoids the formation of rittle aluminum-iron alloys; instead, a strong steel-silveraluminum bond is obtained and a stable silver-aluminum alloy is formed which retains its strength at low and high temperatures.

In the preferred embodiment of this invention, the aluminum oxide surface layer is removed prior to final joint formation. It has been found that if a substantially clean aluminum surface is not available, the resulting bond will be relatively weak. The oxide layer may be removed just before making the joint by, for example, wire brushing the aluminum. However, a thin oxide layer is reformed almost immediately, and to obtain the strongest possible joint, this thin layer should be removed before completing the joint. A convenient and preferred way of accomplishing this is to press-fit the aluminum and stainless steel with a rotary downward motion of the stainless steel member, the pressing and scraping providing the desired cleaning action. Another method is to thread the two metals together. Still another method is to clean the aluminum in an inert atmosphere, e.g. argon, and also carry out the joint formation in the inert atmosphere.

Referring now to the drawings and specifically to Figure 1, the end of stainless steel hollow tube is coated with silver to form a coating 11 of desired thickness and length. Various coating methods well-known to those skilled in the art may be used, although electroplating is preferred. Metal spraying and dipping methods are also suitable. Microscopic examination of the joint crosssection has shown that the aluminum-silver alloy layer is normally about 0.002 inch thick, independent of the total original silver coating thickness. Consequently, the intermediate bonding layer metal should be at least 0.002 inch thick in order to retain enough metal for bonding with the stainless steel. It is preferred that the intermediate metal coating prior to joint formation have a thickness between about 0.004 inch and 0.005 inch. Silver is preferred as the intermediate bonding material, but gold could be used instead.

Aluminum hollow tube 12 is placed in high frequency induction coil 13 and the silver-coated stainless steel tube 10 is preferably placed in contact with one end of the aluminum tube 12 but not inside the heating field of the induction coil 13. The heating unit is then turned on and the aluminum heats the stainless steel tube 10 by conduction. As the aluminum heats up it expands until the stainless steel tube 10 may be pushed down inside of the aluminum tube 12 to the desired depth, as illustrated in Figure 2. The induction heating is then continued long enough to allow a desired amount of diffusion to occur between the silver and aluminum, thus forming a silver-aluminum alloy layer 14 adjacent to and bonded with remaining silver coating 15, as shown in Figures 3 and 4.

The unique advantages of the present invention may be illustrated by the following example describing the formation of a tubular aluminum-stainless steel joint by' the aforedescribed novel process. A type 321 stainless steel tube, 4 inch OD. and 0.0040.005 inch wall thickness, was silver plated on the exterior of one end. The coating was 0.004 inch thick and extended inch from the end of the tube. The coating was polished with emery paper and then washed in alcohol and dried. A inch O.D. aluminum tube was cleaned in a degreasing solution and one end reamed /2 inch deep to form a 0.254

inch I.D. opening. This tube was then placed in a highfrequency induction heating unit so that its top end was about & to inch above the top of a 3-loop induction coil having a /s inch bore. The silver-plated stainless steel tube was placed in contact with the upper end of the aluminum tube. The heating unit was then set to operate at 12 kilowatts for 11 seconds and after about 9 seconds of heating, the stainless steel tube was inserted into the end of the aluminum tube and forced into place with'a rotating downward motion. The resulting bond successfully withstood leak and physical deformation tests. The aluminum tube was then welded to an aluminum shell and the dissimilar metal joint Withstood the high temperatures created during welding without becoming embrittled.

Tensile strength tests of aluminum-silver-stainless steel joints prepared by the process of the present invention indicated that the joint was stronger than the stainless steel base metal since tensile failure occurred first in that member. Aluminum-stainless steel joints produced by prior art methods may be easily pulled apart at the joint due to the presence of the brittle aluminum-iron intermediate compound.

Although the process of the present invention has been described in detail in terms of joining aluminum and stainless steel, it is to be understood that the invention is equally applicable to other combinations of dissimilar metals having the aforedescribed physical property relationships. That is, the projection-containing second dissimilar metallic member must have a higher melting temperature and a higher electrical resistivity, but a lower thermal expansion coefficient than the first hollow section-containing metallic member. Furthermore, the intermediate bonding material must have a melting temperature between the corresponding temperatures for the first and second metallic members to facilitate formation of an alloy between such intermediate bonding material and the lower melting temperature first metallic member.

Materials which could be used instead of stainless steel for the second metallic member of the present invention include ferrous metals, ferrous alloys, nickel, nickel an le.

alloys, and copper alloys useful at low temperatures, such as Everdur. Also, magnesium could be used instead of aluminum as the construction material for the first metallic member.

It is to be understood that certain modifications may be made to the specific embodiments of the invention as disclosed herein, without departing from the scope of such invention. Also, some features may be practiced without utilizing other elements of the invention, all within the aforementioned scope.

What is claimed is:

l. A process for welding together a first metallic member having a first melting temperature, a first electrical resistivity, and a first thermal expansion coeflicient and provided with a hollow section of given contour, and a second dissimilar metallic member having a second melting temperature higher than said first melting temperature, a second electrical resistivity higher than said first electrical resistivity, and a second thermal expansion coeificient lower than said first thermal expansion coetficient, such second metallic member being provided with a projection of contour and size corresponding to said hollow section, said process comprising the steps of coating the projection of said second metallic member with an intermediate bonding material having a third melting temperature intermediate said first and second melting temperatures; establishing a high-frequency magnetic field adjacent to said first metallic member so as to inductively heat and expand such member; inserting the coated projection in the expanded hollow section; and continuing the induction heating until a desired amount of diffusion between the intermediate bonding material and said first metallic member has occurred to form an alloy therebetween.

2. A process for welding together a first metallic member having a first melting temperature, a first electrical resistivity, and a first thermal expansion coefiicient and provided with a hollow section of given contour, and a second dissimilar metallic member having a second melting temperature higher than said first melting temperature, a second electrical resistivity higher than said first electrical resistivity, and a second thermal expansion coeificient lowerthan said first thermal expansion coeflicient, such second metallic member being provided with a projection of contour and size corresponding to said hollow section, said process comprising the steps of coating the projection of said second metallic member with an intermediate bonding material having a third melting temperature intermediate said first and second melting temperatures; placing the coated projection of said second metallic member in physical contact with said first metallic member; establishing a high-frequency magnetic field adjacent to said first metallic member so as to inductively heat and expand such member; inserting the coated projection in the expanded hollow section; and continuing the induction heating until a desired amount of difiusion between the intermediate bonding material and said first metallic member has occurred to form an alloy therebetween.

3. A process according to claim 1 for welding dissimilar metals, in which said first metallic member is constructed from a member of the group consisting of of silver, and gold, comprises said intermediate bonding materiahg 6.-A process according to claim 1 for welding dissimilar metals,1in which said first and second metallic members are constructed from aluminum and stainless steel respectively, and silver comprises said intermediate bonding material.

7. A process according to claim 3 for welding dissimilar metals, in which the oxide coating on the section of said first metallic member to be welded is substantially completely removed before formation of the weld.

8. A process according to claim 6 for welding dissimilar metals, in which the coating of said intermediate bonding material on said projection is at least 0.002 inch thick.

9. A process according to claim 6 for welding dissimilar metals, in which the coating of said intermediate bonding material on said projection is between about 0.004 inch and 0.005 inch thick.

10. A composite welded assembly comprising a first metallic member having a first melting temperature, a first electrical resistivity, and a first thermal expansion coefficient and provided with a hollow section of given contour; a second dissimilar metallic member having a second melting temperature higher than said first melting temperature, a second electrical resistivity higher than said first electrical resistivity, and a second thermal expansion coeflicient lower than said first thermal expansion coefiicient, such second metallic member being provided with a projection of contour corresponding to said hollorw section; a coating of an intermediate bonding material on said projection, such material having a third melting temperature intermediate said first and second melting temperature, the external dimensions of the coated projection being slightly larger than said hollow section at ambient temperature but such that said coated projection tightly fits in the hollow section of the first member when such section is heated; and a welding joint uniting said coated projection and the walls of said hollow section including an alloy of said intermediate bonding material and said first metallic member resulting from diffusion therebetween.

ll. A composite welded assembly according to claim 10, in which said first and second metallic members are constructed from aluminum and stainless steel respectively, and silver comprises said intermediate bonding material.

12. A composite welded assembly comprising a first metallic tube having a first melting temperature, a first electrical resistivity, and a first thermal expansion coefiicient; a second dissimilar metallic tube having respectively a second melting temperature higher than said first melting temperature, a second electrical resistivity higher than said first electrical resistivity, and a second thermal expansion coefiicient lower than said first thermal expansion coefficient; a coating of an intermediate bonding material on one end of the second metallic tube, such material having a third melting temperature intermediate said first and second melting temperatures, the outer diameter of the coated second tube being respectively slightly larger than the inner diameter of the first tube at ambient temperature but slightly smaller than said inner References Cited in the file of this patent UNITED STATES PATENTS Bohner et al.. Mar. 6, 1956 Grenell Nov. 6, 1956 Longacre July 2, 1957 

